The invention concerns a method and a device for determining the quantity of product contained in a reservoir, for example in an ink reservoir for a printer.
In known printing devices, methods have already been proposed for detecting the residual quantity of available ink (this is then sometimes referred to as ink level monitoring or detection).
By way of example the document EP-A2-0 028 399 describes an ink level detection method using an resonant circuit with which the reservoir to be monitored is integrated. More precisely, the capacitor of this resonant circuit has two metal plates forming electrodes defining a dielectric space in which the storage cavity of the ink reservoir is situated.
The ink thus behaves as a dielectric whose value changes as the ink reserve decreases. Consequently the capacitance of the resonant circuit also changes.
The latter is calibrated so that its resonant frequency, and therefore the maximum voltage at its resistor, is reached when the level of the ink reserve has dropped to a predetermined value. When this threshold is passed, a signal is activated.
This method has a certain number of defects.
First of all, when the ink level decreases, the voltage at the terminals of the resistor varies up to a certain threshold. The only information given by this device is therefore an indication on the situation (above or below) of the ink level with respect to the said threshold.
Only the information relating to the amplitude of the measurement signal is taken into account and compared with a threshold; this type of detector indicates only one type of information: when there is 20% of the ink remaining in the reservoir. By using an analogue to digital converter, it would be possible to think of determining the level of ink in the reservoir continuously, but this type of component has a high cost.
In addition, when it is wished to measure the quantity of ink present in small reservoirs or when the capacitance is of low value (a few picofarads), the resonant frequency then becomes extremely high, which appreciably increases the cost of the components used and may generate electromagnetic interference or disturbances. One solution would consist of using inductors of very high value (1 henry for example). These would reduce the frequency, but they are very difficult to produce and in any case are bulky.
It is briefly mentioned in this document that it is possible to use a parallel circuit but it is added that a series circuit is preferable; in any event everything which is described and shown concerns a series circuit.
The ink plotters such as the one mentioned in the description of EP-A-0028399 have a reservoir and a recorder. The reservoir and recorder are not electrically connected to the printing device, which of course makes it possible to place the capacitor consisting of metal plates and ink reservoir in either a series or a parallel configuration.
The same applies to conventional wound inductors, which can be placed equally well in a series or parallel configuration.
It must nevertheless be noted that:
if the recorder is for example connected to a potential, for example earth, the series resonant circuit cannot be produced; this case is however increasingly usual;
if the capacitance is very small, it is necessary to produce inductors with very high values if it is desired to remain within low frequencies, which cannot be achieved in conventional technologies.
A first object of the invention is to mitigate the drawbacks of the known solutions by virtue of a method or device which makes it possible to detect, preferably at least within an operating range preferably including the low values, the residual quantity of an electrically conductive product contained in a reservoir made of an electrically insulating material in a simple and reliable fashion, by identifying an easily measurable characteristic of a resonant circuit including the reservoir, without having to modify the latter in any way, without for all that using expensive components in the processing of the measurement signals.
An other object of the invention (but these aspects can be taken into consideration independently of each other and of the aforementioned object) is to detect the residual quantity of an electrically conductive product contained in a reservoir made of an electrically insulating material in a simple and reliable manner:
even when the technological constraints of location and operation of the system receiving the reservoir allow the formation only of a resonant circuit of the parallel type (in particular when the reservoir includes, or cooperates with, a print head whose operation requires an electrical connection of the said head to a predetermined potential, which prevents any series connection);
at a moderate cost and within a moderate size, in particular without using components which are difficult to produce and/or expensive in the resonant circuit itself or in the generator designed to deliver excitation signals to this resonant circuit);
which easily allows, by means of minor adaptations, the detection also of situations where there is no product in the duct through which the product flows to the print or ejection head (therefore minimizing the additional components to be provided when it is desired not only to detect the quantity of product in the storage chamber but also to check, in real time, that there is indeed, in the delivery duct, product in a normal state, that is to say electrically conductive).
To this end the invention first proposes a method of determining the quantity of an electrically conductive product contained in a reservoir made of electrically insulating material having at least one storage cavity, according to which:
a resonant circuit is formed having a capacitive arrangement comprising at least part of this reservoir;
this resonant circuit is connected to an excitation signal generator;
a measurement procedure is defined according to which a plurality of excitation signals is applied to this resonant circuit having different frequencies included in a predetermined frequency range and a plurality of measurement signals are taken off at a measuring point in response to these excitation signals, this frequency range being chosen so as to contain the frequencies at which the resonant circuit is in resonance for a plurality of values of the quantity of product within a predetermined operating range;
a processing procedure is defined consisting of identifying the instantaneous resonant frequency of the resonant circuit from this plurality of measurement signals, measuring the width, for a given amplitude level, of the resonance peak and deriving therefrom an item of information representing the quantity of product contained in the storage cavity, this amplitude level being chosen so that there exists a univocal correlation law between the width of the resonance peak and this item of information when the quantity of product is within the said operating range;
at least one determination cycle is effected, consisting of triggering the measurement procedure and the processing procedure, and capturing the instantaneous value of the said item of information.
It may be appreciated that the invention thus makes provision for detecting the resonant frequency of a resonant circuit in which a capacitive element consists of at least part of the reservoir, which is easy to achieve at a moderate cost, and to derive a value representing the residual quantity of product from the width of the resonance peak, which requires only the use of a simple comparator, of a cost very much less than that of an analogue to digital converter. However, it became clear that it was possible, for electrically conductive products, notably printing inks, to establish a univocal correlation law between the width of such a resonance peak and the quantity of product at which resonance occurs.
One of the possible sources of cost in implementing the method of the invention lies in the need to be able to generate excitation signals able to result in measurement signals of sufficiently high amplitude to lend themselves to effective and precise use; in other words it is necessary to be able to generate signals at frequencies close to the resonant frequency. Preferably, when possible, the invention is implemented within the range of low or medium frequencies (between approximately 1 kHz and approximately 100 kHz). This can sometimes be achieved easily, having regard to the nature of the product and the geometry and dimensions of the reservoir, using conventional components to produce the resonant circuit.
In the field of printing machines it was seen that the capacitance values typically encountered with ink reservoirs led on the other hand to resonant frequencies in the high-frequency field (beyond around 1 megahertz), unless it was possible to use any inductors of very high values which, when they exist, are very expensive.
The advantage of using, according to a preferred characteristic of the invention, a notional inductor is that it is possible to simulate high-value inductors easily without using components which are complex or difficult to produce. A so-called xe2x80x9cgyratorxe2x80x9d circuit is thus known which, with a few resistors and two amplifiers, makes it possible to simulate a high constant inductance using a constant capacitance of conventional value (typically around scarcely a few picofarads) of moderate cost and bulk. However, it became clear that installing such resistors and such amplifiers entailed in itself only a moderate increase in cost and bulk, so that such a gyrator lent itself very well, in spite of appearances, to forming notional inductors of high value at a cost and within an overall bulk which were entirely moderate, including when operating in the field of office printing machines.
It is consequently easy to design the resonant circuit so that it has an average resonant frequency (between around 1 kHz and around 100 kHz), and so that the excitation signals necessary for detecting the quantity of product can therefore be generated very easily at a moderate cost.
Preferably, the capacitive arrangement of which the reservoir (or a part thereof) forms part is distinct from the notional inductor.
It may be noted that the method lends itself very well to a parallel connection of the capacitive and inductive components of the resonant circuit, which makes it applicable to any type of reservoir, whatever the type of associated ejection or print heads. For reasons of simplicity or to meet operating constraints, these components are advantageously connected between a measuring point and earth.
When the reservoir has several storage cavities in series, the method of the invention can be applied to monitor one or more of these cavities. In particular, when the reservoir has a first cavity containing a porous body and a second cavity communicating with the delivery duct through the first cavity, it may be preferred to dispose the electrode designed to be in the immediate vicinity of the chamber of the reservoir opposite at least part of this second cavity: thus the residual quantity of product is monitored in the whole of the reservoir. It is of course possible to monitor only part of the reservoir by disposing for example the aforementioned electrode opposite the first cavity.
Preferably, the capacitive arrangement includes two metallic parts forming the electrodes of a capacitor, one of which is disposed in the immediate vicinity of and opposite a portion of the storage chamber of the reservoir, and the other one of which is formed by, or connected to, an ejection or print head connected to the storage chamber by a connecting or delivery duct, by virtue of which the capacitive arrangement takes account not only of the quantity of product in the chamber but also in the connecting duct. Such an assembly makes it possible to add, to the parts necessary for the operation of the head, only a single metal part.
The operation of certain print heads currently known makes it necessary for the latter to be connected to earth: this is why it is may be preferred, or even necessary, the resonant circuit then being of the parallel type, for the print or ejection head to be connected to a reference potential formed by earth.
The invention applies notably to the case of printing machines using a reservoir, generally removable, containing an electrically conductive ink: the resonant circuit, including the first electrode, is then advantageously fixed with respect to the casing of the printing machine.
It is very easy to adapt the method of the invention for monitoring the state of the product in the delivery duct, whether there is a lack of it, or whether it dries up, notably. The characteristics of the capacitor formed by the reservoir between the electrode and the print head are then fundamentally modified, leading in practice to values of the characteristic (or of the measurement signal) which are entirely different from the values which can normally be obtained: it suffices to provide for a test in this regard and an abnormality procedure (excitation of an audible or light signal for example) to be triggered as appropriate.
It is clear that the information concerning the quantity of product can be of at least two natures, depending on whether concern is with the quantity already consumed or the residual quantity.
The choice of the amplitude threshold at which the width of the resonance peak is determined has an influence on the operating range within which it is desired to monitor the change in the quantity of product. When the amplitude at the resonance increases as the residual quantity of product decreases, it is found that the width of the peak also increases, and, the more the threshold represents a high percentage of the maximum amplitude at the resonance for a substantially empty reservoir, the narrower the operating range, close to very low values. Preferably, the threshold is chosen so that the operating range contains at least values of the quantity of product representing from 0% to 60% of the maximum quantity of product (the aim is then to measure the variation in ink level in the critical range corresponding to the end of the cartridge as far as 0%). The level of the amplitude threshold is preferably chosen to as to be equal to approximately 50% of the maximum possible amplitude of the resonance peak (that is to say for an xe2x80x9cemptyxe2x80x9d reservoir with a minimum quantity of product).
The excitation signals are preferably alternating signals, but can also, in a variant, be square-wave or pulsed signals.
The invention also proposes, for implementing this first method, a device for determining the quantity of an electrically conductive product contained in a reservoir made of electrically insulating material, having:
a resonant circuit including a capacitive arrangement designed to comprise at least part of this reservoir, this circuit having, for various possible values of the quantity of product contained in a given operating range, resonant frequencies contained within a predetermined frequency range;
an excitation signal generator connected to the resonant circuit and adapted to generate various frequencies belonging to this predetermined range;
measurement and processing means connected to this resonant circuit and to the excitation signal generator and designed so as to apply to the resonant circuit a plurality of excitation signals having various frequencies within the said predetermined range, to detect a measurement signal in response to each excitation signal, and to measure the width, for a given amplitude level, of the resonance peak and to derive therefrom an item of information representing the quantity of product contained in the storage cavity, this amplitude being chosen so that there exists a univocal correlation law between the width of the resonance peak and this information when the quantity of product is within the said operating range; and
means for capturing the instantaneous value of the said information.
The invention also proposes a method of determining the quantity of an electrically conductive product contained in a reservoir made of electrically insulating material, this reservoir having a storage chamber and a product delivery duct and being provided with an electrically conductive ejection head connected to this duct, according to which:
a resonant circuit is formed, having an arm including a capacitor comprising at least part of this reservoir and an arm including a notional inductor, these arms being connected in parallel between a measuring point and a reference potential, this capacitor having two conductive elements forming electrodes, one of which is in the immediate vicinity of a zone of the storage chamber and the other one of which is formed by the said ejection head;
this resonant circuit is connected to an excitation signal generator;
a measurement procedure is defined, according to which at least one excitation signal is applied to this resonant circuit and a measuring signal is taken off the point of measurement in response to each excitation signal;
a processing procedure is defined, consisting of identifying a characteristic of the resonant circuit from this measurement signal or signals and deriving therefrom an item of information representing the quantity of product contained in the storage cavity, this characteristic being chosen so that there is a univocal correlation law between this characteristic and this information;
at least one determination cycle is effected, consisting of triggering the measurement procedure and the processing procedure, and capturing the instantaneous value of the said item of information.
It may be noted that the method provides for a resonant circuit whose capacitor and inductor are connected in parallel, which makes it applicable to any type of reservoir and any type of ejection or print head.
Most of the above comments apply here.
The characteristic of the resonant circuit from which according to the invention there is a derived a measurement of the quantity of product contained in the reservoir can be the amplitude of the measurement signal obtained for a fixed frequency. This is because, in order to ensure that this amplitude varies univocally over a wide range of values of the quantity of product, it is preferable for this frequency to be substantially equal to the resonant frequency of the resonant circuit when the quantity of product is extreme, that is to say preferably maximum or possibly minimum. This is because, the lower the quantity of product, the higher the resonant frequency, but the higher this frequency, the lower the amplitude of the resonant frequency: the choice of a fixed frequency close to the resonant frequency for an extreme value of the quantity of product guarantees that the instantaneous resonant frequency of the circuit remains constantly either less than or greater than this fixed frequency.
It may nevertheless turn out to be more precise to seek to be positioned, at each measurement, at the instantaneous resonant frequency of the resonant circuit since the amplitude of the measurement signal is at a maximum there (for a given quantity of product). However, it appeared that the resonant frequency varied univocally with the quantity of product: it is therefore possible to choose the instantaneous resonant frequency as a characteristic of the resonant circuit. However, it appeared that the amplitude at the resonance also varied univocally as a function of the quantity of product: this amplitude can therefore also be chosen as a characteristic of the resonant circuit. It then suffices to provide for the measurement procedure to include a preliminary step consisting of identifying a range in which the resonant frequency of the resonant circuit varies when the quantity of product varies, and for various measurement signals to be taken off for various frequencies of this range; the frequency at which the amplitude of the measurement signal has a maximum value is taken as a measurement of the instantaneous resonant frequency whilst the amplitude at this frequency is taken as a measurement of the amplitude at the resonance.
The functioning of certain print heads currently known requires these to be connected to earth; this is why it may be preferred, or even required, for the reference potential to which the print or ejection head is connected should be that of earth. It is clear that the means for earthing the ejection head may be any means, more especially as, as has just been noted, this connection of part of the head to earth is already often effected independently of the invention.
The invention also proposes, for implementing the method, a device for determining the quantity of an electrically conductive product contained in a reservoir made of electrically insulating material, this reservoir having a storage chamber and a product delivery duct and being provided with an electrically conductive ejection head connected to this duct, having:
a resonant circuit having a first arm designed to include at least part of the reservoir and a second arm including a notional inductor, these arms being connected in parallel between a measurement point and a reference potential, this first arm having an electrical conductor disposed in the immediate vicinity of a zone of the storage chamber and a means for connecting the said ejection head to the reference potential, this first electrical conductor and this ejection head defining a dielectric space containing at least part of the storage chamber and the delivery duct and thus forming a capacitor;
an excitation signal generator connected to the resonant circuit;
measurement and processing means connected to the said measurement point and to the excitation signal generator and designed so as to apply at least one excitation signal to the resonant circuit, to detect a measurement signal in response to each excitation signal, to identify a characteristic of the resonant circuit from this measurement signal or signals and to derive therefrom an item of information representing the quantity of product contained in the storage cavity, this characteristic being chosen so that there is a univocal correlation law between this characteristic and this item of information;
means for capturing the instantaneous value of the said item of information.
The same comments as before apply to the device thus defined.
The invention also concerns a signal processing device formed by measurement and processing means and means for capturing the instantaneous value of the information sought.
The invention applies finally to:
a product supply device combining a device for determining the quantity of product with the reservoir and the means of controlling the ejection head;
the particular case, important in practice, where this product supply device is an image forming device;
a printing system including only the device for determining the quantity of product with the reservoir, in the case of an ink reservoir;
an office machine including any one of the aforementioned devices; and
an office signal processing unit designed to cooperate with an ink reservoir and including a processing device of the aforementioned type;
a means, fixed or partially or totally removable, of storing information which can be read by a computer or microprocessor storing instructions of a computer program, characterised in that it enables the methods of the invention as briefly disclosed to be implemented, and
a means, fixed or partially or totally removable, of storing information, which can be read by a computer or a microprocessor storing data resulting from the implementation of the methods as briefly disclosed above.
It will be appreciated that the invention makes it possible notably:
to establish a univocal relationship between the quantity of ink remaining in the reservoir and the frequency band at which the voltage is above a predetermined threshold (or a univocal relationship, approximately linear, between the remaining quantity and the resonant frequency of the RLC circuit, in the case of the second method);
to detect at what moment there is no ink in the pipe, since the delivery channel is situated in the dielectric space defined by the metal parts;
to produce a level-measurement system integrated into the print device and requiring no modification to the ink cartridge;
to reduce the bulk of the mechanical configuration by using the print head as the second plate of the capacitor;
to use low frequencies by using a circuit of the gyrator type as an inductor.